The present invention relates to ultrasonic flowmeters of the type that measure time of flight of an ultrasonic signal transmitted and received alternately between a pair of oppositely disposed transducers.
Various types of flowmeters have been utilized for clinical applications, that is, for the measurement of the velocity of gases flowing within a flow system administering oxygen, nitrous oxide and an anesthetic agent to a patient. After an accurate flow velocity is obtained, various other parameters necessary to properly monitor the gases to and from a patient are readily calculated, including tidal volume, breathing rates and the like.
Such flowmeters have included vortex-shedding type, rotameters featuring various shaped rotating vanes or blades, hot wire anemometers, but all have certain disadvantages due to the particular need for accuracy over various flow ranges and in the presence of a particular environment.
Since relative low pressures are experienced in the patient's breathing circuit, the flowmeter must be designed so as to present a minimum resistance to breathing and introduce minimal pressure drop in the system. Further qualifications of such flowmeters are that they must be easily cleanable or, in the alternative, be isolated completely from the flow of gas in the patient circuit to prevent cross-contamination between patients. A relatively fast response time is needed so that attending personnel can keep an accurate and instantaneous record of the conditions of gases supplied to or from the patient. As such, therefore, the flowmeter should also have bidirectional capability.
The conditions in a patient circuit also make the task of such flowmeters difficult, the density of the gas stream is subject to changes as the concentration of anesthetic gases is changed and the actual gas stream makeup is variable, i.e., the flow velocity must be determinable both in the presence and in the absence of various chemical substances and gases.
One viable solution to the aforementioned difficulties in operating conditions has been the use of ultrasonics in flowmeters. Ultrasonic transducers are commercially available and are capable of acting both as transmitters and receivers. Such flowmeters may be of the single transducer type utilizing Doppler frequency shift measurement or time-of-flight measurement which utilize more than one transducer and measure the actual time elapsed as the transmitted signal travels from one transducer to the other. The former approach may have some inherent disadvantages, in that it does not measure a time average velocity but may interpret peak velocities. Also, the Doppler technique requires a flow media that is an efficient reflector of the acoustic energy and such is not normally the case in patient breathing circuits.
Accordingly, the latter principal has been proposed and used for measuring flow velocity in patient circuits. See, for example, the article by Plaut and Webster entitled "Ultrasonic Measurement of Respiratory Flow, IEEE Transactions on Biomedical Engineering," Vol. BME-27, No. 10, October 1980, and the flowmeter relies upon the principal that velocity of the fluid medium itself has an effect on the time of flight of the ultrasonic pulse. The sound takes longer from transmitter to receiver when traveling upstream in the flow medium as when traveling downstream, therefore by placing the transducers opposite each other about the flow path of the gas to be measured, but at an angle to the mean flow path, one can derive both an upstream component and a downstream component.
In the equation ##EQU1## as derived by Plaut and Webster in the aforementioned publication, it is possible to measure the shift in phase angle between the ultrasonic signal emitted by the transmitter transducer and the signal detected by the receiver transducer and thus obtain an accurate analysis of the transit time of the ultrasonic signal traveling therebetween.
One difficulty, however, as expressed by Plaut and Webster, is the inherent ambiguity when the phase angle difference exceeds 360.degree.. Normally, to measure the phase angle shift, one establishes a phase reference at zero flow and measures the shift recognized by the receiving transducer. When that phase shift exceeds 360.degree. the flowmeter cannot readily distinguish the true phase angle shift and therefore the result is ambiguous.
As a practical example, it is known that gas density affects sound velocity and in clinical applications, it is common to have a gas stream to be analyzed that contains both air and nitrous oxide. The sound velocity of air at 20.degree. C. is 342 meters/sec. and of nitrous oxide is 273 meters/sec. For a separation between transmitter transducer and receiving transducer of 25 mm or greater, the zero flow phase shift at 100 kHz is at least 660.degree. and is thus beyond the capability of present ultrasonic flowmeter systems, using the phase measurement approach.